The availability of data on plasma vitamin D (25(OH)D) concentrations in the last two rounds of the National Health and Nutrition Examination Survey has generated a high level of interest in the consequences of 25(OH)D deficiency and in particular its potential impact on black Americans. Fundamental new questions about the biology of vitamin D have now come to the fore. Are low 25(OH)D-intact parathyroid hormone (iPTH) levels a physiologic abnormality in African Americans or does the 25(OH)D-iPTH system have sufficient plasticity to adapt to low sunlight exposure? Could the low 25(OH)D levels which result from the gene- environment discordance observed in dark skinned populations in the US be a risk factor for metabolic disorders? Ultimately, the public health community and regulatory bodies must offer recommendations for optimal levels and supplementation. We propose to utilize cohorts from an ongoing NIH-funded study designed to identify determinants of weight change and cardiovascular disease risk in five Afro-origin populations [Modeling the Epidemiologic Transition Study (METS)] to examine these questions. The five METS countries include Ghana, Seychelles, South Africa, Jamaica and the US; the five populations differ greatly in terms of sun exposure and dietary intakes. In addition to the extensive energy expenditure, dietary intake and obesity-related metabolic markers being measured in METS, we propose to assess plasma 25(OH)D, iPTH, serum and urinary calcium, as well as bone mineral density and body composition using dual x-ray absorptiometry. The associations of adiposity, weight change, CVD risk factors such as blood pressure and insulin, and bone mineral density with 25(OH)D and iPTH will be assessed. The proposed study will provide a comprehensive assessment of the distribution and determinants of 25(OH)D-iPTH and related physiologic measures across a wide range of latitude and lifestyles. These data should contribute substantially to our understanding of the normal range within which these hormones function and their significance in Afro-origin populations.